The present application relates generally to mobility spectrometers for measuring particle size distributions in fluid streams. More specifically, the present application is directed to a fast integrated mobility spectrometer, spectrometer and method of measuring participle size distributions of a wide size range.
Real-time measurement of particle size distributions, especially in the nanometer size range, is important in many applications such as measurement of atmospheric aerosols and characterization of particles in combustion systems. Rapid measurements are often required to capture transient aerosol dynamics occurring on very small time scales, such as in high temperature environments or other nucleation-dominated systems. In other types of measurements, such as those onboard fast-moving platforms (e.g., research aircraft) aimed at characterizing spatial and temporal distributions of atmospheric aerosols, high time resolution is essential to capture the variations of aerosol properties over small spatial domain.
Currently, sub-micrometer aerosol size distributions are often measured using a Scanning Mobility Particle Sizer (“SMPS”). The SMPS is a sequential measurement technique. Therefore, only particles within a narrow size range, which represent a small fraction of total particles introduced, are measured at one time. Obtaining the entire aerosol size distribution requires scanning the classifying voltage over a wide range, which typically takes about one minute and is too slow for aircraft-based measurements. The time required for scanning the classifying voltage can be reduced by using a fast-response detector (e.g., mixing-type condensation nucleus counter). However, because only a small fraction of total particles is measured at a time, the sampling rate of the SMPS is insufficient for rapid measurements. As a result, despite the improvement in measurement speed by using the fast-response detector, measurements in clean environments are often compromised by the time required to obtain statistically significant numbers.
Other mobility-based instruments have been used to measure particles of different mobilities simultaneously. For example, Electrical Aerosol Spectrometer (“EAS”), Engine Exhaust Particle Sizer (“EEPS”) and Differential Mobility Spectrometer (“DMS”) have been used to measure particles of different mobilities simultaneously using an array of integrated electrometers, capable of sub-second measurements of aerosol size distributions. However, due to the low sensitivity of the electrometers, applications of these instruments are limited to aerosols with high number concentrations, such as engine exhausts. Besides low sensitivity, the EAS, EEPS, and DMS also have a considerably lower size resolution than does the SMPS.
Another instrument frequently used to measure sub-micrometer aerosol size distributions is an Optical Particle Counter (“OPC”), which measures particle sizes based on the intensity of light scattered by the particles. The OPC offers fast measurement speed and better counting statistics than does SMPS, but its measurement range is usually limited to particles with diameters greater than 100 nm. In addition, particle physical properties such as shape, refractive index, and morphology have strong influences on derived particle sizes, and are often unavailable. Even for the ideal case of homogeneous spherical aerosol particles, the uncertainty in refractive index often leads to significant uncertainties in derived size distributions.
A Fast Integrated Mobility Spectrometer (“FIMS”) has been developed previously to measure particles of different mobilities simultaneously. See U.S. Pat. No. 7,298,486 to Wang, et al. The disclosure of the '486 patent is incorporated herein by reference in its entirety. The simultaneous measurement of particles with different sizes/mobilities provides size spectra of sub-micrometer aerosol at a time resolution of 1 Hz, nearly 100 times faster than traditional SMPS. Since individual particles and their mobility dependent positions are detected optically using a high resolution CCD camera, the FIMS also offers high size resolution and counting statistics. While the FIMS is capable of rapid measurements, it has a relatively narrower dynamic size range compared to SMPS. To achieve good size resolution, the measurement range of a single FIMS is limited to about a decade in electrical mobility. For the measurements of sub-micrometer size distribution ranging from about 15 nm to about 1000 nm, three FIMS can be operated in parallel, with each of the FIMS operating at a different separating voltage and covering a fraction of the total size range.